Previously, AT&T customers who owned iPhones had to be connected to Wi-Fi to make calls via the Internet, and other smartphone users were out of luck.

Developers will need to modify their VOIP apps to take advantage of the capability, which will take some time. But some seem to have gotten a head start: Vonage, an Internet-calling service, recently rolled out an app which uses both Wi-Fi and AT&T's 3G network.

AT&T has been notoriously guarded about its 3G bandwidth -- and customers have been outraged by network slowdowns and dropped calls. The company has been concerned that too many VOIPusers may overload their data network. Critics say it was more about protecting profits by disabling apps that allowed free phone calls.